A few years ago we discovered a rather surprising effect of short-term adaptation to visual contrast, an effect we now call the Straddle Effect (although originally nicknamed “Buffy adaptation”). After adapting for less than a second to a grid of evenly-spaced Gabor patches all at one contrast, a test pattern composed of two different test contrasts can be easy or difficult to perceive correctly. When the two test contrasts are both a bit less (or are both a bit greater) than the adapt contrast, observers perform very well. However, when the two test contrasts straddle the adapt contrast (i.e. one of the test contrasts is greater than and the other test contrast is less than the adapt contrast) performance drops dramatically.

To explain the Straddle Effect, we proposed a shifting, rectifying contrast-comparison process. In this process a comparison level is continually updated at each spatial position to equal the recent (less than a second) weighted average of contrast at that spatial position. The comparison level is subtracted from the current input contrast, and the magnitude of difference is sent upstream but information about the sign of that difference is lost or at least degraded.

In this previous work the test pattern and the observer's task were of the type known as second-order. We began to wonder: Is that necessary? As it turns out, the answer is “no”. Here we will show a temporal Straddle Effect with a single Gabor patch having contrast that varies over time (in a two-temporal-interval same/different task). Thus the shifting, rectifying contrast-comparison process may occur in both spatially first-order and second-order vision. The important quantity in human contrast processing may not be something monotonic with physical contrast but something more like the un-signed difference between current contrast and recent average contrast.